The present invention relates to a mobile communication system employing a code division multiple access mode having radio logical channels such as control channels and traffic channels, and more particularly to a mobile communication system and a base station in which a channel efficiency is enhanced by the processing of canceling an interference ascribable to radio logical channel signals contained in a received signal from another mobile station.
A mobile communication system employing a code division multiple access mode has features suited for mobile communications, such as immunity against a cross-correlation interference from another system, immunity against multipath fading, easy handoff, and superior security protection. In the code division multiple access mode, however, the users of a plurality of mobile stations existing within an identical base station cell communicate sharing an identical frequency band, unlike in other multiple access modes. Therefore, the code division multiple access mode has a problem that interferences are liable to occur among the mobile stations. This problem is incurred by the interference ascribable to the phenomenon in which the intense communication signal of the mobile station user near a base station conceals another weak communication signal, the interference between mobile stations by unnecessary power transmitted from at least one of them, and so forth.
Meanwhile, as countermeasures preventive of lowering in a channel efficiency, it is taken to utilize a broadband gain by widening the band of signals, to use code sequences whose cross-correlations are weak, to adopt a power control for solving a near/far effect, and so forth. Especially the power control is effective for the reason that the transmission power of each local mobile station is suppressed to the required minimum, thereby to minimize the interference between mobile stations, and also to solve the near/far effect.
Even with such countermeasures, however, increase in the cross-correlation interferences (degradation in a signal-to-noise ratio) is inevitable due to increase in the number of users who are simultaneously communicating. Therefore, it has heretofore been considered difficult to prevent the lowering of a communication quality attendant upon the cross-correlation interferences.
In a base station reception apparatus employing the code division multiple access mode, reception signals to become cross-correlation interference components from other receivable users are predictable signals, and it is not impossible to cancel the cross-correlation interference components by signal processing based on the prediction of the reception signals. Therefore, interference canceling techniques which cancel the cross-correlation interferences from the other users have been deemed potential as means for solving the problem ascribable to the interferences as stated above. By way of an example, an interference canceling technique of parallel multistage type has been studied as stated in JP-A No. 7-131382.
In the present situation, however, the application of interference cancellation to an actual system, in other words, to a system accompanying a call control application operation, has not been proposed though the interference cancellation itself has been studied.
The processing of a common control channel is necessitated in order that the call control application operation may proceed in the actual system. The common control channel is employed for systemic information notification at the initial stage of a communication start, and system notification information is obtained after the demodulation of the common control channel. Therefore, a time period longer than one radio frame is expended on the identification of the spread codes of the common control channel.
According to an interference cancellation scheme in the mobile communication system which employs the code division multiple access, the cancellation processing of the cross-correlation interferences is executed in such a way that the replica of a received user signal is generated and is subjected to signal processing. In making the common control channel a subject for the interference cancellation, therefore, it is necessary to solve the problem concerning a delay time which is involved in the demodulation of the common control channel.
The delay involved in the demodulation cannot be ignored in performing a high-speed power control. It is accordingly necessary to propose a novel system which implements the high speed power control and permits the interference cancellation processing.